Base load circuit for mechanical rectifiers



April 21, 1959 G. KLIESCH v 2,883,602

BASE LOAD CIRCUIT FORMECHANICAL RECTIFIERS Filed March 29, 1955 IN V EN TOR. Gwv 7E2 K1. lESC/l A rme/vm United States Patent Ofiice 2,883,602 Patented Apr. 21, 1959 BASE LOAD CIRCUIT FOR MECHANICAL RECTIFIERS Giinter Kliesch, Erlangen, Germany, assignor, by mesne assignments, to Siemens-Schuckertwerke A.G., Berlin and Erlangen, Germany, a corporation of Germany Application March 29, 1955, Serial No. 497,744 5 Claims. (Cl. 321-48) My invention relates to a base load circuit for mechanical rectifiers and more specifically to a base load circuit which by-passes the main operating contact.

Mechanical rectifiers to which my novel invention applies are clearly described in copending applications such as Serial No. 483,497 filed January 24, 1955, now Patent No. 2,782,360, and U.S. Patent No. 2,693,569, assigned to the assignee of the present invention. Mechanical rectifiers in general consist of three major components. First is a power transformer, the second, commutating reactors, and the third, mechanically driven cooperating contacts. Rectification is accomplished by synchronously opening and closing the cooperating contacts with a mechanical linkage driven by a synchronous motor which is connected to the AC voltage to be rectified. The commutat-ing reactor which is a saturable type reactor is connected in series with the contacts and operates to create a low current step during the time the contact is being opened and closed.

Since the contacts in a mechanical rectifier which is rectifying a sixty cycle source operate 216,000 times per hour, the function of the commutating reactor to maintain a low current step 1y important one.

However, in starting a mechanical rectifier, the flux of the commutating reactors is in an undetermined state and will not unsaturate at the proper time for at least the first cycle of operation. Therefore, the commutating reactors will not operate in synchronism with the cooperating contacts, and the contacts will not be protected by the low current steps of the commutating reactors during contact make and break operation.

If now the main load is energized after the synchronous motor is started to operate the contacts, the contacts being unprotected by the commutating reactor low current step for the initial operating cycles, may engage or disengage on currents high enough to destroy the contacts.

This problem has been overcome in the past by providing a high impedance dummy load or base load in the DC. side of the rectifier and in series with the contacts to replace the main load until the starting process is finished.

The high impedance base load then served to limit the current through the unprotected contacts when the rectifier is started and until the commutating reactors work in synchronism with the cooperating contacts. After this synchronous condition is reached, it has been the practice to make final adjustments in auxiliary equipment and the contacts while the rectifier is operating on the base load and prior to the energization of the main D.C. load.

However, even the high impedance base load circuits used in the past must carry as high as thirty amperes to assure proper operation under base load conditions. Since the base load is in series with the cooperating contacts, it is possible that the full magnitude of this current, that is, currents up to thirty amperes will be made and broken by the contacts.

As is well known in the art, mechanical rectifier contacts can be damaged when operated on currents of this magnitude even if only for a few cycles. Furthermore,

during contact operation is a vital- 2 damage caused during starting on base load conditions is accumulative and could lead to complete destruction of the contacts after either a few starts or after only one start and a short period of normal operation.

Another disadvantage of this circuit is that the adjust ment of auxiliary equipment and contacts operation frequently leads to backfire or short circuits between two phases due to improper contact timing or faulty adjustment of the auxiliaries.

The principle of my invention is to substitute auxiliary rectifying means for each of the main contacts and connect these auxiliary rectifying means to a circuit base load for the base load operation. By so doing, I can then allow the commutating reactor low current step production to become synchronized with the main rectifier contacts without forcing the main rectifier contacts to carry any of the starting base load current. I can, for instance, accomplish this by placing the load in series with a dry ,cell rectifier of each phase of the mechanical rectifier which is connected between the commutating reactor and its series connected contact. This base load circuit would therefore have its own auxiliary rectifier and would be ,energized to thereby insure proper energization of the commutating reactors before the main D.C. load, which is in series with the main rectifier contact, is energized. Furthermore, the main contacts would be brought into synchronous operation but would carry no current since there is no return path (the main D.C. load is open) through the main cooperating contact.

Furthermore, adjustments on base load can be made without fear of backfire since misadjustment of the commutating reactor protective step or contact timing will not result in a short circuit between phases because the rectifying means replacing the main rectifier contacts can have inherently unidirectional characteristics.

In the following, I shall describe my novel base load circuit in conjunction with a dry cell rectifier which is substituted for the main cooperating contacts of the rectifier during starting conditions. It should be noted, however, that my novel circuit could use electronic rectifying means or an auxiliary set of contacts to replace the main cooperating contacts of the rectifier during base load operation.

Accordingly, a primary object of my invention is to substitute an auxiliary rectifying means for the main coopcrating contacts of the mechanical rectifier during base load operation to thereby prevent current flow through the main contacts during base load operation.

Another object of my invention is to provide a base load circuit for mechanical rectifiers which does not include the main rectifier contacts.

Still another object of my invention is to provide a base load circuit for mechanical rectifiers which will not cause backfire due to adjustments made under base load operation.

A further object of my invention is to provide a base load circuit for mechanical rectifiers which has an auxiliary rectifier connected at a point between the commutating reactor and the main rectifier contacts to supply a base load circuit.

Mechanical rectifiers are adjusted for proper contact operation and proper adjustment of all auxiliary equipment during the base load operation. Very often, however, the rectifier will backfire during the course of the adjustment; that is, a fault may occur between two phases. Backfires in mechanical rectifiers usually result in the destruction of one or more contacts. However, by using my novel base load circuit in which the contacts carry no current, fear of backfires due to base load adjustment is eliminated.

Accordingly, another object of my invention is to provide a base load circuit in which adjustment and observation of contacts and auxiliary equipment will not lead to backfires.

These and other objects of my invention will become apparent from the following description taken in connection with the drawing which shows my novel base load circuit taken in conjunction with a six phase full wave mechanical rectifier.

The choice of this type connection is an arbitrary one and my novel base load circuit has universal application to all mechanical rectifier circuits. During normal operation, power from A.C. line it) is transformed in the power transformer 11 and subsequently supplied as DC. power to the main DC. load 12. The DC. load 12 is connected to the rectifier by means of disconnect switch 22 and circuit breaker 23. Rectification is accomplished by the main rectifier contacts 13 and 14 which are protected by series commutating reactors 15 and 16. The base load circuit to which my novel invention is directed comprises current limiting resistor 17, current smoothing choke 13, switch 19 and rectifiers 20 and 21.

It should be noted that rectifiers 20 and 21 comprise a self-contained rectifier unit and carry the DC. output of the rectifier when switch 19 is closed and DC. load 12 is deenergized by opening switch 22 and circuit breaker 23'.

The operation of the circuit is as follows and, for the sake of simplicity is taken in conjunction with only the phases marked A and A.

Circuit breaker 23 is open to thereby deenergize load 12. The contacts 13 and 14 are being driven by a motor and linkage which is not shown here, but is described in the above mentioned applications. The main rectifier transformer 11 is then energized from the AC. source 10 and switch 19 is closed. A current will now flow through commutating reactor 15, rectifier 20, base load resistor 17 and inductor 1%, switch 19, rectifier 21, commutating reactor 16, and back to the AC. source.

It is important to note that there is no current path through the main mechanical rectifier contacts 13 and 14 since circuit breaker 23' is open. With the flow of this DC. current, the fiux in commutating reactors l5 and 16 is automatically synchronized to provide low current steps in synchronism with the opening and closing of contacts 13 and 14 although these contacts do not carry the energizing currents. Rather, their rectifying function is accomplished by means of the auxiliary rectifiers 20 and 21. As soon as rectifier operation under base load conditions is found to be satisfactory, and all necessary adjustments are made, it is now possible to energize the main D.C. load 12 by closing circuit breaker 23 and subsequently opening the base load switch 19. Since the contacts are now driven normally and commutating reactor operation has been assured due to operation on the base load circuit, the transfer of load current from the high impedance base load circuit to the low impedance load will be effected without difficulty and the contacts 13 and 14 will be immediately provided with low current steps by the preenergized commutating reactors 15 and 16.

Therefore, the use of my novel base load circuit prevents current flow in the main contacts during the interval in which they would not be protected by low current steps of their associated commutating reactors, and assures synchronization of the commutating reactor low current step with the main contact operation before the main load is energized.

Furthermore, after base load operation is achieved, ad ustments may be made in the rectifier, such as contact time adjustment and observation of bouncing contact operation without fearing contact destruction due to backfires or fault conditions arising from improper'contact operation.

Although I have disclosed a preferred embodiment of my invention, it will now be apparent that many modifi cations and variations may be made by those skilled in the art. Therefore, I prefer to be limited not by the specific disclosure herein but only by the appended claims.

Iclaim:

1. In a mechanical rectifier for energizing a direct current load from an alternating current source; said mechanical rectifier comprising cooperating contacts in series with a commutating reactor; said alternating current source, commutating reactor, cooperating contacts and direct current load connected in series when said cooperating contacts are engaged; a base load impedance and an auxiliary rectifying means; said base load impedance, auxiliary rectifying means, alternating current source and commutating reactor connected in series; said rectifying means constructed to cause direct current energization of said base load impedance and having the polarity desired for full load operation.

2. In a mechanical rectifier for energizing a direct current load from an alternating current source, said mechnnical rectifier comprising cooperating contacts in series with commutating reactors; said alternating current source commutating reactors, cooperating contacts and direct current load connected in series when said cooperating contacts are engaged; a base load circuit connected in parallel with said cooperating contacts and direct current load; a rectifying means; said base load circuit, rectifying means and commutating reactor connected in series; said rectifying means connected to supply voltage of the polarity desired for full load operation.

3. A base load circuit for mechanical rectifiers supplying a direct current load from an alternating current source by means of mechanically driven contacts comprising a base load impedance and a self contained rectifier unit; said base load circuit connected to replace said mechanically driven contacts and said direct current load during base load operation of said mechanical rectifier.

4. A base load circuit for mechanical rectifiers, said mechanical rectifiers energizing a direct current load from an alternating current source, and having cooperating contacts and a commutating reactor, said direct current load, commutating reactor, cooperating contacts and alternating current source connected in series when said cooperating contacts are engaged; said base load circuit comprising a base load impedance and a self-contained rectifier; said base load circuit connected to force said alternating current source to supply direct current power to said base load impedance in series with said cooperating contacts and to completely energize said commutating reactors in synchronism with said alternating current source, said base load circuit connected in parallel with said cooperating contacts and direct current load.

5. In a three phase mechanical rectifier to energize a direct current load from a three phase alternating current source; each phase of said mechanical rectifier comprising a series connected commutating reactor and mechanically driven cooperating contacts; each phase of said mechanical rectifier forming a series connection of said alternating current source, commutating reactor, cooperating contacts and direct current load when said cooperating contacts are engaged; auxiliary rectifiers and a base load impedance, each phase of said mechanical rectifier comprising a second series connection of said alternating current source, commutating reactor and said base load impedance; said series connected auxiliary rectifiers and base load impedance connected in parallel to said mechanically driven cooperating contacts and direct current load; said rectifying means connected to cause direct current energization of said base load impedance having the polarity desired for full load operation.

References Cited in the file of this patent UNITED STATES PATENTS 2,622,234 Blatter Dec. 16, 1952 

